Heat exchanger surfaces are most often thermally energized by a fuel fired means opposite the surface which is contacted by the fluid to which the heat is to be transferred. One example of such an exchanger is a tube which is externally energized and through which the fluid to be heated is passed. Another example is a diaphragm or wall having one surface to which thermal energy is imparted and an opposite surface which is contacted by the liquid or gas to be heated.
In many processes where thermal energy is used, it is desirable to monitor the temperature of the surface to which the thermal energy is applied. However, the surface which receives the thermal energy is also the surface which is exposed to the condition which is most hostile to the exchanger material. The purpose of monitoring this surface is to provide an accurate signal that is representative of the temperature thereof so that maximum process conversion can take place without overheating the exchanger material and thereby degrading the exchanger. It will thus be understood that it is important to maintain a constant thermal energy input to the exchanger where the highest conversion rates are obtained without exceeding the limits of the exchanger material, and this condition can only be provided using temperature sensors having the highest accuracy and reliability.
In the past the temperature of heat exchanger surfaces were ordinarily monitored by means of thermocouples. However, the use of thermocouples has necessitated the use of expensive signal converting equipment and reference junctions as well as special alloy leads which connect the thermocouple to the other parts of the system.
Resistance temperature detectors, commonly called RTDs, have been used for making accurate temperature measurements. They utilize a resistance element whose resistance changes with the ambient temperature in a precise and known manner. The resistance temperature detector may be connected in a bridge circuit which drives a display calibrated to show the temperature of the resistance element. However, the useful life of an RTD is shortened if the resistance element is subjected to extremely high temperatures such as the ambient temperature on the high temperature side of a fired heat exchanger where temperatures in excess of 1,800 degrees Fahrenheit are commonplace. Also, it is extremely important to isolate the resistance element from direct contact with the products of combustion which would otherwise alter the resistance stability of the resistance element and thereby adversely affect the accuracy of the RTD system.